Hi Jen252!
Looking at you're list of specs and wishes for the PC, I don't think you'll get something for the £700 budget you mention which will 'last' well.
Photo editing and then as you say moving onto video editing are some of the most intense tasks you can ask a computer to do - in order to get good performance you are going to need better quality/performaing components. I get the impression this PC will be used for business purposes so it's even more imperitive you have a fast, capable, solid working machine.
There are PC's around the £700 mark which will go a good way in providing what you'd like, however the sticking point is that you say you'd like a small case. Small cases and usually dual graphics cards (SLI configuration) don't usually mix well - you'll need at least a 'normal' sized tower case - and some of the power house PC's aimed at photo and video editing are larger still as you must consider power supply requirements and cooling.
I'm kinda in the same position as you - as I'd like a compact unit to run Microsoft Small Business Server 2003 at home - so don't want a large noisy 'usual' Server - I want something quiet and unobtrusive and the choices are very very narrow.
The Quad core Q6600 is a good processor and nowadays cheap in ready made PC's. Take a look at the MESH Elite Q4 FX at £699 inc VAT. I'd ask for an SLI upgrade and a 2nd HD upgrade, and maybe you could delete the Monitor if you already have a decent one? I'd say for photo editing you'd need a 24 inch 1920x1200 panel. This Mesh system sits at the bottom end of their performance range, and of course, the Midi tower maybe realistically the smallest you can go for the use you need from it.
I bought 2 Mesh PC's for the business about a year ago, one for web development work and one for our graphics designer guy - at the time I bought just base units, with a Core 2 Duo 6700 processor, and SLI config 2x256MB graphics cards and 2 GB RAM for the web guy and 4GB RAM for the graphics machine. I think I paid something like £850 each for them.
Seeing as you'll be moving into video editing, that where you'll need raw performance and grunt which I'd say really means you'd need a larger tower case for the cooling needs - the power supply will need to be a higher rated unit to cope.
If you want to try to 'get away'' with using a smaller SFF or even shuttle case have a look at these guys
http://www.theglowlounge.com/catalog/I got really happy as they'd provide me a shuttle sized system filled to the gills with top end components, but I configured what I wanted and currently it's out of my budget! - I haven't used them as yet, but for a system like this, I don't want to use a cheaper guy in his bedroom type deal, I want somewhere I can go back to in case there's problems later.
Also, i'm fast coming to the conclusion, that wanting a small or ultra small PC and trying to run hot and powerful components is going to have an impact on reliability and by using the system more, those demands will be higher and thus reducing its reliabilty even more. I'm therefore going to resign myself to running a server on at least a 'normal' sized PC case and then maybe use some extra fans or water cooling to help with performance boosts etc.
Finally, have a look here
http://www.pcpro.co.uk/labs/94247/pc-nextday-zoostorm-57501-advanced-pc.html?searchString=photo+edit+pcI can't find it right now, but a few issues back they did a review of about 8 Photo Editing PC's - right up your street in terms of needs - none as far as I can remember were in SFF or Shuttle sized cases for the reasons above. See if you can sift through the site or find another magaziens website where they've done comparisons for similar PC's.
I'm also going to say, if you do want to 'get the small system and run it and decide it's worth the risk in potential reduced stability' then go right ahead - it's difficult to gauge what project you will be doing, they might even mean the machine doesnt break a sweat! in which case you'll be happy and have all that you want although I still suspect at a higher than £700 price tag!
Good luck! if you need any more info i'd be happy to help
Cheers, DtM! West London & Slough UK!
P.S - the sound card, extra USB slots, wi-fi options can all be easily added in and are secondary to you main needs - could even go external solutions too- but internal upgrades are simple for these elements.
P.P.S - When I was evaluating I also looked at Evesham, but with their recent rather dodgy history I'm definitley not recommending them. One better place I found and talked to is
www.cube247.co.uk these guys will build some fantastic specced machines and it was a close call between them and mesh when I ordered, these guys only lost out as they said they needed 5 days to build the PC whereas mesh could do it in 3, otherwise I think I'd have gone with these guys. UK company and UK based support which I liked. The other's I found were 'too small' of a business and really served local markets - there may be other specialist system builders out there, but these are the ones I've been happy when speaking to them on a pre-sales level